90 MINERALOGY AND LITHOLOGY. 



section can be obtained, should be referred ; and thus the species pos- 

 sess sufficiently distinctive properties. But in fine-grained aggregates, 

 where the nature of a feldspar can be only approximately determined, 

 and where the difference between these kinds of feldspars is of no litho- 

 logical importance, plagioclase is a good general term for use, and it has 

 become so rooted in lithology that it is liable to maintain its foothold, 

 though perhaps in this modified sense. It seems that species of feldspar 

 may grade into one another as other minerals do, where no theory is con- 

 sidered necessary to account for it ; so, as referring to a subgroup of the 

 triclinic feldspars, it must be granted that the word plagioclase is con- 

 venient, and in this sense it will be used. The three kinds of plagioclase 

 will be treated of as distinct species whenever the composition of a feld- 

 spar is known with certainty, and plagioclase will refer to imperfect or 

 approximate determinations. The question, whether these three mem- 

 bers of the plagioclase group should be merged into one mineralogical 

 species, is of httle practical importance, so long as the subject is so well 

 understood as at present. 



57. Anortiiite [Ca AP Si' O']. 



This kind of feldspar is a constituent of some of our diabase rocks and 

 greenstones. It is contained in some of the calcareous diorites in the 

 Connecticut valley, but the most notable specimens are found in the 

 diabase at East Hanover. There a rock occurs that is filled with large 

 crystals of this species. They are dull on their surfaces, but they pos- 

 sess quite a number of planes, the prominent ones being the base, the 

 domes 21, 2 1, and the prismatic faces I and ii; and some edges are 

 rounded off as if an effort were made to form other planes. These crys- 

 tals are often an inch in length and breadth. Some are flat, and others, 

 by a greater development of the prismatic planes, are thick and short. 

 The rock is so full of them that it is an anorthite porphyry. 



These crystals are no less interesting because they have undergone an 

 almost complete alteration. Before referring to this, it will be well to 

 recall that anorthite is commonly subject to the polysynthetic twinning, 

 which makes its base striated, and its sections, in polarized light, banded. 

 A basal section, when jDlaced between crossed Nicols, with its bands par- 

 allel to the plane of vibration of the light, must be revolved through a 



